Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for measuring the oxygen saturation level of arterial blood. Pulse oximeters perform a spectral analysis of the pulsatile component of arterial blood in order to determine the relative concentration of oxygenated hemoglobin, the major oxygen carrying constituent of blood. By providing early detection of decreases in the arterial oxygen supply, pulse oximetry reduces the risk of accidental death and injury. As a result, pulse oximeters have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care.
A typical pulse oximeter comprises a sensor and a monitor. The sensor has emitters and a detector and is attached to a patient at a selected tissue site, such as a fingertip or ear lobe. The emitters project light at red and infrared (IR) wavelengths through the blood vessels and capillaries of the tissue site. The detector is positioned so as to detect the emitted light as it emerges from the tissue site. The signal generated by the detector is proportional to the intensity of the detected light. The detector signal has a variable (AC) component due to light absorption by the pulsatile volume of arterial blood. The detector signal also has a constant (DC) component due to light absorption by the non-pulsatile volume of arterial blood, venous blood, bone and other tissue. A signal processor inputs the detector signal and determines oxygen saturation and pulse rate, which are typically shown as a numerical readout on a display. In addition to oxygen saturation and pulse rate (PR), some pulse oximeters measure perfusion index (PI), which is a relative indication of pulse strength at a monitoring site. In particular, PI is the ratio of the IR AC signal to the IR DC signal. A pulse oximetry sensor and monitor are described, respectively, in U.S. Pat. No. 6,088,607 entitled Low Noise Optical Probe and U.S. Pat. No. 5,482,036 entitled Signal Processing Apparatus and Method, both of which are assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.